If you’re an eye doctor you know about this stuff. If you’re not, it’s ok to get fluorescein “die” in your eye from the eye doctor. I’m referring to the topically applied orange colored little strips of paper that kinda looks like those pH test strips. But both of you hang on because I have a few interesting things to say about this commonly used diagnostic chemical. It sounds horrible…chemical….but it’s not at all horrible otherwise we wouldn’t use it.

What’s interesting about these strips with fluorescein is that they fluoresce. And pray tell what does that mean? This is where I get to talk about quantum mechanics again! If you didn’t read the last post, it all started there. This little piece of paper, the fluorescein strip when touched to your eye, will turn a different color when blue light is shined on it.

So here’s what I’m saying. The color of the fluorescein is orange on the strip. It goes in your eye orange. Blue light is turned on. The fluorescein in your eye no longer looks orange. It’s green! Turn the blue light off and the fluorescein in your eye is orange again.

Here’s where quantum mechanics comes in. Easy lesson here. The blue light has a certain wavelength, it’s shorter, and thus has more energy. It hits the orange fluorescein in your eye. The little electrons in the fluorescein absorb that energy but a cool property of something that fluoresces is those electrons return some of the energy. And the energy we see is less than the blue wavelength. The color looks green! Green has a longer wavelength than blue. And this is the case with most things that fluoresce. The higher energy of light causes the fluorescing material to “reflect” a lower energy wavelength. You might wonder what happened to the rest of the energy. It is also released by those electrons that sent out the green wavelengths. The remaining energy is heat. A pretty limited amount so there’s no heat you can feel. There’s a measurement of the ability of a material to fluoresce which is called quantum yield and fluorescein has a high yield which kinda means we see the green reflection very easily.

Let’s just take a made up example of fluorescence. Say you shine green light at a material that fluoresces. What are the likely wavelengths it would “reflect?” Right, maybe yellow or orange or red. Those have less energy than green.

But not all things fluoresce enough for us to see, obviously. If they did the world would be very psychedelic. But there are a lot of substances that when a certain wavelength of light hits them, they send back a different wavelength that we can see. Black lights (aka, ultraviolet lights) are very cool because there are a lot of things that fluoresce from that wavelength of energy. There are all kinds of things that fluoresce from UV light like some rocks. I read on a website that an apartment property manager was going to take an ultraviolet light to the carpet of a renter to see if there’s any urine from pets there. Yep, urine fluoresces under UV light. Better not try to hide your pets. Property managers have their “secret” ways to find out!